The invention relates to radio receivers operable for communication in a radio system in which information is transmitted in bursts at periodic intervals in accordance with a system clock, a burst having a field enabling the radio receiver to synchronise therewith. Particularly, but not exclusively, the invention relates to such telephones for communication with base stations in a digital cellular radio telephone system employing transmission by a plurality of carrier frequencies in frames, such as TDMA frames, each consisting of a predetermined number of time slots.
In a radio telephone system, a handset must be in contact with the infrastructure of the system in order for it to make and receive calls. In order to communicate with the system, a handset must be listening to signals transmitted by the system and respond to those signals in an appropriate manner. Typically radio telephone systems allocate a plurality of channels for transmitting and receiving information.
In order to be in a position to make and receive calls the handset must monitor the system to be able to receive and decode messages transmitted. Once the handset has located a suitable channel for monitoring system information and perhaps indicated its presence, a periodic monitoring of the system information is sufficient to locate paging signals used to alert a handset to an incoming call. In this condition the handset is determined to be `locked on` or in `idle lock`.
Communication channels will typically have a predetermined frequency and in TDMA systems will also be allocated a specific time slot for communication. It is important that a handset is able to transmit and receive information at appropriate times in order for the information to be decoded by a receiver. The process of determining the appropriate time for receiving and transmitting signals is known as synchronisation.
In digital systems, information is typically transmitted in bursts. The information in each burst is formed from a number of bits. The information derived depends on both the bit value and the position of a particular bit within a burst. To achieve synchronisation and be able to receive and transmit information, the handset must transmit and receive signals at appropriate times. This necessitates achieving both bit and burst synchronisation for transmitting and receiving. Bit synchronisation for receiving a transmitted signal involves locating the positions of respective bits of a message within a burst so the information content can be determined. Burst, or slot synchronisation involves determining the position within a burst at which a particular bit or string of bits is occurring so that the type of information a particular string of bits contains is known.
Once synchronisation has been achieved the handset sets its internal clock to adopt the same timing as the system clock to enable subsequent transmissions on the located channel to be monitored. It also enables the handset to transmit signals to the system at the appropriate times. To maintain synchronicity with the system and allow the handset to check for incoming calls, amongst other things, the handset will typically monitor the channel periodically, waking before necessary to receive a burst on the channel being monitored, and resynchronising with the transmitted signal to ensure information is correctly received and to compensate for drift between the internal clock of the handset and the system clock. Because of this drift it is important that the point at which the handset `wakes` to monitor a located channel has a tolerance built in so that the handset does not miss the beginning of a burst as this may give the handset insufficient opportunity to synchronise with a received burst.
The built in tolerance, however, creates a problem as, in order not to miss the beginning of a burst even if there is significant drift between the internal and system clocks the window of time in which the handset looks for a burst on the channel it is monitoring is desirable relatively large. The larger the window, however, the more opportunity for the handset to lock on to a noise signal instead of the synchronisation field of the transmitted burst. If a handset locates a noise signal and locks on to that, the handset will adjust its internal timing to accord with the supposed timing of a system transmitting the noise signal and subsequently be unable to relocate the channel it was monitoring as the internal clock will no longer be compatible with the actual timing of the system. If, on the other hand, the window of time in which the handset looks for the start of a burst is relatively small to reduce the chance of synchronising with noise, the possibility of missing a burst on the channel is increased. If the transmission of a burst is missed on several consecutive occasions the internal clock of the handset may have drifted so far relative to the system clock that it is unable to relocate a burst on the channel it is monitoring. Having lost that channel the handset would have to reinitiate the locking on procedure with the system which both increases power consumption and the chance of missing a call. More apparent to the user would, however, be the loss of an existing call or other communications link.